The present invention relates to a method for preparation of polyolefin oils, and particularly relates to a method for preparation of polyolefin oils which can practically or perfectly prevent halogen from being present in the polymerization product and provide polyolefin oils of high viscosity index at a high yield.
The art of polymerizing olefins in the presence of Lewis acid such as aluminum chloride is well known. However, there have been quite a few instances where a polyolefin oil qualified for use as lubricant and the like was successfully prepared industrially by this method. The reason is that polyolefin oils useful for lubricant such as grease, gas turbine engine oil, hydraulic fluids for aircraft, etc. or textile finishing agent or cosmetic base are required to have a relatively low viscosity and a high viscosity index, whereas it is very difficult to obtain polyolefin oils meeting this requirement at a high yield by the use of the well-known Lewis acid catalysts. It is admittedly possible to obtain a polyolefin oil with low viscosity, subject to effecting polymerization at a high temperature, even by the use of Lewis acid. However, this method is defective in that isomerization of the olefin would concur with the polymerization reaction, whereby not only the viscosity index of the polymerization product lowers but also the yield decreases, and a lot of halogen contained in the catalyst gets mixed in the polymerization product.
The presence of halogen mixed in the polymerization product has a bad influence of grave importance on the process of aftertreatment of the polymerization product. For instance, at the time of removing the unreacted olefin and/or olefin dimer from the polymerization product by distillation, there takes place the thermal cracking of a part of the halogen in the polyolefin oil to give rise to hydrogen halide, entailing such a trouble that the distillation apparatus gets corroded thereby. Furthermore, at the time of saturating double bonds remaining in the polymerization product for the purpose of improving the oxidation stability and/or thermal stability thereof, there occurs such a trouble that the halogen spoils the hydrogenation catalyst and, to cite an extreme case, the hydrogenation catalyst is perfectly disqualified for reuse. Therefore, this method has a drawback that the ratio of the cost of hydrogenation treatment to the cost of manufacturing the polyolefin oil becomes extraordinarily high.
Under such circumstances, there has so far been proposed a method comprising polymerizing lower grade olefins by the use of a catalyst prepared by mixing aluminum chloride with metallic aluminum powder or metallic zinc powder thereby to prevent halogen from getting mixed in the polymerization product and at the same time improve the yield compared with the case wherein aluminum chloride is exclusively employed as catalyst. This method admittedly renders it possible to obtain a polyolefin oil containing no halogen at a reasonable yield as long as the polymerization is effected at a relatively high temperature. However, even in this method, the viscosity index of the resulting polymerization product is low like in the case of employing aluminum chloride exclusively as catalyst, and the yield is not necessarily satisfactorily high.
In U.S. Pat. No. 3952071, there is disclosed a method of preparing an olefin oligomer having a low viscosity and a high viscosity index. In this method, a mixture of a polyhydric alcohol derivative and aluminum chloride, said derivative being obtained by replacing the hydrogen atoms of all the hydroxyl groups of the polyhydric alcohol either by acyl groups exclusively or by acyl groups and alkyl groups, both of said groups having 1 to 20 carbon atoms, is used as polymerization catalyst. According to this method, an olefin oligomer scarcely containing halogen can admittedly be obtained at a good yield, but no suggestion is given thereby as to the use of ester of polycarboxylic acid as one ingredient of a catalyst mixture.